1. Field of the Invention
The invention relates to a rotary die cutter adapted to effect a punching and scribing on a sheet which is fed between a die cylinder and an anvil cylinder, the die cylinder being adapted to be rotated in a predetermined direction and provided on its peripheral surface with a knife and a scribing rule, while the anvil cylinder disposed in parallel with the die cylinder being provided on its peripheral surface with an anvil for receiving the knife and the scribing rule and being adapted to be rotated in the opposite direction to the die cylinder, and more particularly to a construction for compensating for secular change of quality of the anvil surface due to the punching by the knife.
2. Description of the Prior Art
FIGS. 1 and 2 show schematically the construction of a conventional rotary die cutter. Referring to FIG. 2, a sheet 9 transferred from a preceding step of process is forwarded in the direction of arrow over a sheet guide 8 and fed into the gap between an anvil cylinder 1 and a die cylinder 2 by means of a pair of feed rolls 6, 7. The anvil cylinder 1 is provided with an urethane rubber anvil 3 wound therearound, while a die 71 attached to the outer periphery of the die cylinder has a knife 5 for punching the sheet and a scribing rule 70.
The anvil cylinder 1 and the die cylinder 2 are adapted to be rotated in the opposite directions as indicated by arrows. The clearance between these cylinders is adjusted such that the edge of the knife 5 cuts into the anvil 3 to a suitable depth. The radial height of the scribing rule 70 is slightly smaller than that of the knife 5 so that a slight gap is formed between the end of the scribing rule 70 and the anvil 3. The scribing rule 70 is adapted to form in the sheet 9 a recess or groove (referred to as "scribed line", hereinafter) which is used as the reference when the sheet 9 is folded. The sheet 9 is completely cut by the knife 5 as the latter cuts into the anvil 3 across the sheet 9, so that the anvil 3 is inevitably damaged by the knife 5. The damage in the surface of the anvil 3 gradually develops and grows as the operation is continued over a certain period, so that the anvil surface is undesirably roughened so that the cutting of the sheet 9 is locally deteriorated or hindered. In such a case, the distance between the axes of the cylinders is adjusted to obtain a sufficiently large cutting of the knife 5 into the anvil 3. As this process is repeated, the outside diameter of the anvil 3 is gradually reduced due to the cutting by the knife and the roughness of the anvil surface is further increased. In consequence, the punching of the sheet 9 by the knife 5 is made incomplete. In addition, since the gap between the anvil surface and the scribing rule 70 is made non-uniform, the depth of the scribed line is rendered also non-uniform. In the worst case, the sheet is completely penetrated by the scribing rule 70 at some portions of the scribed line. This in turn poses a problem that the sheet cannot be folded precisely when it is folded into the shape of the box.
In addition, the feed speed of the sheet 9 is gradually lowered partly because of the reduced peripheral speed of the anvil 3 due to decreased diameter and partly because of a reduced friction between the sheet 9 and the anvil surface due to the roughening of the anvil surface. When the anvil 3 has been worn out to have extremely large roughness on the anvil surface, the anvil is renewed with a new anvil.